橡胶材料的非线性黏弹性本构方程

橡胶是一种非线性黏弹性材料,准确描述其非线性黏弹性力学响应的本构方程是橡胶材料及制品设计优化的关键。文中基于超弹性模型和并行流变模型(PRF)描述了橡胶材料的非线性黏弹性响应特征,重点探讨了由实验数据确定PRF本构方程材料参数的方法。首先通过单轴拉伸实验数据拟合得到超弹性模型,将应力松弛实验数据拟合得到表征材料线性黏弹性的模型-Prony级数,再将Prony级数转化为初始的PRF模型,进而不断优化得到PRF模型的准确材料参数,最后进行实验验证。结果表明,PRF模型计算的不同应变下的应力松弛数据与实验数据之间的误差仅为0.067%,PRF能准确地描述橡胶材料非线性响应的应力松弛行为。     

生物可降解材料PLGA的应力松弛性能

针对基于生物可降解材料聚乳酸-羟基乙酸共聚物(PLGA)的热压成型工艺仿真研究中,粘弹性材料模型参数缺乏的问题,通过单轴拉伸应力松弛试验,研究了生物可降解材料PLGA在5种温度状态下的应力松弛性能。基于时间-温度等效原理,平移得到60℃的松弛模量主曲线,并采用广义Maxwell模型对此主曲线进行拟合。结果表明:随着温度的升高,PLGA材料的松弛时间明显缩短,且拟合曲线与原数据误差较小,说明广义Maxwell模型能较理想地模拟PLGA材料的应力松弛行为。这些数据的获得为后续仿真研究提供了参考。     

聚合物应力松弛的非定常微分型本构模型

利用粘滞系数随时间变化的粘性元件和弹性模量随时间变化的弹性元件,构造非定常(也可称为变参数)Maxwell模型和非定常Zener模型。求解非定常模型的本构方程得到它们的松弛函数。结果表明,当粘滞系数和弹性模量随时间按幂律规律变化时,可以把经验函数stretched exponential函数和修正的stretched exponential函数视为非定常模型的应力松弛函数。文中用修正的stretched exponential函数对聚甲基丙烯酸甲酯(PMMA)和聚四氟乙烯(PTFE)松弛模量实验数据进行了拟合,表明该函数能较好地描述这两种聚合物的应力松弛。     

聚合物应力松弛的非定常微分型本构模型EI北大核心CSCD

利用粘滞系数随时间变化的粘性元件和弹性模量随时间变化的弹性元件,构造非定常(也可称为变参数)Maxwell模型和非定常Zener模型。求解非定常模型的本构方程得到它们的松弛函数。结果表明,当粘滞系数和弹性模量随时间按幂律规律变化时,可以把经验函数stretched exponential函数和修正的stretched exponential函数视为非定常模型的应力松弛函数。文中用修正的stretched exponential函数对聚甲基丙烯酸甲酯(PMMA)和聚四氟乙烯(PTFE)松弛模量实验数据进行了拟合,表明该函数能较好地描述这两种聚合物的应力松弛。     

一种新的聚合物基复合材料应力松弛经验模型

视黏弹性元件弹簧和粘壶的特性参数为时间的函数,建立双变参Maxwell模型,求解其本构方程及松弛函数,探讨了其松弛函数与经验KWW函数的关系,提出了一种新的应力松弛经验模型。运用最小二乘原理,建立该模型参数的确定方法。开展了玻璃纤维增强聚合物(GFRP)复合材料的长期应力松弛实验,并通过实验数据的分析验证了该模型的正确性。研究表明,新应力松弛经验模型拟合曲线与实测曲线吻合较好,相关系数达到96%以上,说明了该模型适合描述GFRP复合材料的黏弹性松弛特性。     

有机玻璃热压弹性回复实验与建模

首先通过热压缩实验研究热压工艺参数对有机玻璃(PMMA)热压后弹性回复的影响。从实验结果可知,温度和作用时间是影响热压后弹性回复率的重要因素。然后引入广义麦克斯韦方程作为热压应力松弛的控制方程,通过实验结果和模型计算结果对比,由该方程拟合的热压应力松弛曲线和实验测得的曲线具有较高的吻合度。最后由麦克斯韦方程导出弹性回复的数学模型,通过与实验结果对比可知,该数学模型对弹性回复的预测的误差小于10%。     

高聚物PETG的应力松弛行为的实验研究与分数阶模型

通过实验研究了玻璃态高聚物PETG材料在不同温度、跨越屈服点的不同初始应变的非线性应力松弛行为。有关PETG实验研究进一步补充了当前该材料相应条件下实验数据的缺乏,也为该材料应力松弛行为的进一步研究提供参考。同时,文中在Scott-Blair模型的基础上添加等效时间项,推导出一种用以描述PETG复杂应力松弛行为的分数阶模型。使用最小二乘法拟合模型获得了不同条件下的模型参数,结果表明,屈服和接近玻璃化转变温度对应力松弛行为有相似的影响。模型分数阶阶数是决定整个应力松弛过程的决定性参数。进一步对模型参数进行全局灵敏度分析,结果从数值上解释了分数阶阶数变化与其表征的应力松弛关联密切的原因。     

天然胶松弛曲线的一种拟合方法

采用单一的Maxwell模型确定被测材料的平均松弛时间,适当选择各运动单元的松弛时间,建立了五参数数学模型,各参数均用最小二乘法拟合。结果表明,均方根偏差均小于3％,松弛谱基本呈正态分布。     

废弃高分子材料的内应力消除及使用寿命预测

为了减少废弃饮料瓶片(回收聚对苯二甲酸乙二酯,rPET)与瓶盖(回收线性低密度聚乙烯,rLLDPE)共混物的内应力,采用反应挤出法制备新型增容剂rLLDPE与甲基丙烯酸缩水甘油酯接枝乙烯-辛烯共聚物(POE-g-GMA)的接枝共聚物,通过溶剂沉浸法和热焓松弛拟合来观察和计算共混物的链段松弛时间,并利用动态力学性能预测材料的使用寿命。结果显示共混物在60℃下热处理25h以后,样条应力发白现象减弱,与热焓松弛拟合结果一致。120℃下热处理10h后的共混物在130~180℃时符合时温等效原理和WLF方程。根据共混物老化活化能的变化确定老化临界失效时间,利用寿命方程计算得到共混物在25℃下的使用寿命为9.6年。     

牛顿法拟合动脉瓣的应力松弛函数

依据实验数据，用最小二乘法拟合了人主动脉瓣的粘弹性性能参量——归一化应力松弛函数．这是一十非线性规划问题．在假定数学模型的条件下，我们推导了牛顿法的全部公式，井在计算机上进行迭代．获得了良好了的结果．最终的应力松弛量大约为20％。     

复合材料螺栓连接预紧力松弛的温度-时间依存行为

针对碳纤维增强环氧基复合材料机械连接结构耐久性设计中的2个关键问题:黏弹性预紧力松弛的温度-时间依赖行为及其长期性能预测方法进行研究。建立了以蠕变全应变理论为基础的预紧力松弛预测模型。36 h的恒温耐久试验表明:初始预紧力越大,温度越高,连接件预紧力松弛速率越快;复合材料连接件的松弛速率远大于金属连接件;预紧力松弛主要表现为材料蠕变过程。对比短期试验结果表明:本模型能较好地实现对不同温度、预紧力和连接材料的松弛预测,为确立试验数据的外推方法提供了依据。      

BTG塑料合金准静态力学性能研究

为研究BTG塑料合金的准静态力学性能,并建立准确描述其力学性能的分数导数模型,通过DMA242进行了20、40、60、80和100℃下1000 s时长拉伸松弛实验,以此实验数据为基础,根据时温等效原理获得了20℃下440 000 s时长的时间松弛模量.以此识别参数获得了BTG塑料合金的分数导数Kelvin本构模型,同时...     

基于应力松弛实验对服役25Cr35Ni型耐热钢的高温性能评估

以服役的25Cr35Ni型钢为对象,研究利用应力松弛实验开展高温性能评估的方法以及对持久性能的预测效果。结果表明:由于高温服役后晶界处碳化物出现网链状和奥氏体基体内二次碳化物明显粗化,25Cr35Ni型耐热钢持久性能降低。通过得到的不同温度和应力的松弛蠕变速率曲线及外推关系,结合松弛蠕变速率-断裂时间关系方程,可以实现由松弛实验及少量蠕变持久实验开展持久寿命评估。与基于高温持久实验的预测结果比较,两者吻合较好。     

Use of the Wilshire equation to correlate and extrapolate creep rupture data of Incoloy 800 and 304H stainless steel

Advanced power plant alloys must endure high temperatures and pressures for durations at which creep data are often not available, necessitating the extrapolation of creep life. A recently developed creep life extrapolation method is the Wilshire equation, with which multiple approaches can be used to increase its goodness of fit to available experimental data and improve the confidence level of calculating long-term creep strength at times well beyond the available experimental data. In this article, the Wilshire and Larson–Miller parameter equations are used to extrapolate the creep life of Incoloy 800 and 304H stainless steel to 100,000 h. The use of (1) different methods to determine creep activation energy, (2) region splitting, and (3) short-duration test data to predict long-term failure were investigated to determine their effects on correlation and extrapolation using the Wilshire equation. The accuracy of the Wilshire and Larson–Miller parameter equations’ calculated times to rupture are compared.     

Stress Relaxation of Nonlinear Thermoviscoelastic Materials Predicted from Known Creep

A method to predict the stress relaxation response of nonlinear thermoviscoelastic materials from known creep data is presented. For given nonlinear creep properties, and creep compliance represented by the Prony series, it is shown that the Schapery creep model can be transformed into a set of first order nonlinear differential equations. By solving these equations the nonlinear stress relaxation curves for different strain and temperature levels are established. The strain/temperature-dependent constitutive equation can then be constructed for any nonlinear thermoviscoelastic model, as needed for engineering applications. This revised version was published online in July 2006 with corrections to the Cover Date.     

Experimental Study and Numerical Modelling of Creep and Stress Relaxation of Dielectric Elastomers

Dielectric elastomers (DEs) are gaining acceptance as potential actuator materials because of their exhibition of a large amount of deformation when stimulated by electrostatic forces. However, time‐dependent behaviour such as creep and stress relaxation still pose a great challenge for the design, modelling and control of the DE‐based actuators. In this work, attempts are made for experimental estimation and modelling of creep and relaxation properties of one of the most widely used dielectric acrylic elastomers, VHB 4910. Experimental investigation shows that the material possesses strong time‐dependent creep and stress relaxation. It has been shown that creep and stress relaxation characteristics vary with the holding stress and holding strain respectively. Creep and stress relaxation properties are also shown to depend on the number of cycles in the case of cyclic loading. Results also show that Findley's power law can successfully model the creep and stress relaxation behaviour of the VHB 4910 elastomer.     

Prediction of the behaviour of copper alloy components under complex loadings by electro-thermomechanical coupled simulations

ABSTRACT

Electronic devices must be served with electric power for different reasons. A robust and reliable electric connectivity is often realised by electric connectors. For its leading properties, precipitation hardened copper alloys are widely used for designing connectors with high level mechanical or conductance properties. However, copper alloys show a characteristic stress relaxation under mechanical or thermal loads. Finite element analysis is a standard method to design and optimise components with respect to reliability and performance. Hence, a material model considering the characteristic of the mechanical properties and allowing for the simulation of time and temperature dependent elasto-viscoplastic material behavior was developed at the Fraunhofer IWM. The parameters of the model were determined using tensile and relaxation test data of a C19010 alloy. The material model is applied to electro-thermomechanical coupled finite element simulations of connectors with different load histories. The goal of the simulations is the analysis of the impact of stress relaxation on the mechanical properties of systems over time. From the numerical results with the new model it is shown, how stress relaxation influences the connector clamping forces or contact pressure, respectively, in dependence with time or temperature. The simulation results documents that stress relaxation has to be taken into account in finite element simulations during the designing process of electrical devices.

This paper is part of a Thematic Issue on Copper and its Alloys.     

Proposed fatigue damage measurement parameter for shot-peened carbon steel based on fatigue crack growth behavior

A new technique for evaluating fatigue-damage accumulation in shot-peened (SP) carbon steel based on variations in residual stress is proposed. Using findings from previous studies, a fatigue damage parameter for a material treated with SP based on the change in induced compressive residual stress (CRS) is examined. A plastic replica method with the focused ion beam (FIB) technique is used to assess the relationship between the residual stress state and the fatigue crack growth (FCG) behavior of SP specimens over the fatigue lifespan. It is found that the residual stress relaxation phenomenon can be used as an effective parameter for determining the fatigue damage growth, provided the residual stress relaxation rate of each mechanical load and the critical threshold relaxation boundary of each material is known.